This invention relates to acid gas loading within an amine solution and, more particularly, to determining hydrogen sulfide (H2S) and carbon dioxide (CO2) loading within the amine solution by utilising liquid conductivity to determine the loading of the hydrogen sulfide and carbon dioxide.
Contracts are entered between the sellers or producers of natural gas and the transmission companies who transport such gas within their pipelines. These contracts specify the parameters of the gas sold by the producers that will be transported within the pipelines of the transmission companies. The prices for transportation of such gases are based on the specifications and, indeed, the gas eventually transported within the pipeline is required to fall within appropriate limit parameters.
Sweet or sales gas is carried by the pipelines. This gas is the originally produced sour gas which has had a certain percentage of hydrogen sulfide and carbon dioxide removed. The sour gas emanates from the producing formation of the oil or gas well. It is transported to an absorber containing an amine solution. The acid gases chemically react with the amine and are removed in their liquid form. This so called xe2x80x9csweet gasxe2x80x9d from the absorber is subsequently dehydrated and sold to the transmission company.
The hydrogen sulfide and carbon dioxide removed from the sour gas leave the absorber within a xe2x80x9crichxe2x80x9d amine solution which has a relatively high concentration of the acid hydrogen sulfide and carbon dioxide. This rich amine solution is introduced into a regenerator or stripper and a reboiler. As the rich amine flows through the regenerator and reboiler, the hydrogen sulfide and carbon dioxide is stripped from the amine. The resulting xe2x80x9cleanxe2x80x9d amine containing relatively low concentrations of hydrogen sulfide and carbon dioxide is recirculated back into the absorber where it is reused on the sour gas flowing into the absorber as just described.
A parameter that is important for controlling the reversible reaction both in the absorber or contactor and the stripper or regenerator is the concentration measurement or the xe2x80x9cloadingxe2x80x9d of the hydrogen sulfide and carbon dioxide in both the lean and rich amine solutions. In respect of the lean amine loading calculation, a feed forward indication is provided to the absorber which is used to calculate the quantity of lean amine solution which must be provided to the absorber so the appropriate quantity of acid gas is removed from the sour natural gas and to ensure the sweet or sales gas meets the pipeline specifications. Lean amine calculations further provide a feed back indication of how efficiently the regenerator is stripping the acid gas from the rich amine liquid. In respect of rich amine liquid, a feed forward indication for the calculation is provided which will provide the quantity of energy necessary within the regenerator/reboiler to strip the acid gas from the rich amine liquid.
Heretofore, the measurement for amine loading of the amine solution used to remove the acid gases from the production sour gas was obtained by driving the acid gas components of the liquid amine into the gaseous phase and then analysing the gases. An ultraviolet gas analyzer was used for the hydrogen sulfide and an infrared analyser was used for the carbon dioxide. The results of the two analyzers were expressed as a concentration measurement, conveniently parts per million (ppm) or grains/gallon.
While this technique has certain advantages such as confining the concentration measurements to the two specific gases, namely carbon dioxide and hydrogen sulfide, and thereby avoiding interference with any other gases in the gas stream, the use of two analyzers is expensive, there is a large lag time to obtain the measurement because of the phase change requirement and the maintenance costs for two analyzers and their associated components are high.
According to one aspect of the invention, there is provided a method for determining the acid concentration of an amine solution carrying hydrogen sulfide and carbon dioxide comprising the steps of determining the conductivity of a first liquid stream containing said hydrogen sulfide, said carbon dioxide and heat stable salts in said amine solution; removing significantly all of said heat stable salts from said first liquid stream to form a second liquid stream; determining the conductivity of said second liquid stream containing said hydrogen sulfide and said carbon dioxide without said heat stable salts; removing significantly all of said hydrogen sulfide from said second liquid stream to form a third liquid stream; determining the conductivity of said third liquid stream containing said carbon dioxide without said hydrogen sulfide; and analysing said conductivity measurements of said first, second and third liquid streams to obtain said acid gas loading of said amine solution.
According to a further aspect of the invention, there is provided, apparatus for determining the acid concentration of an amine solution carrying hydrogen sulfide and carbon dioxide comprising a first analytical cell for measuring the conductivity of a first liquid stream containing said hydrogen sulfide, said carbon dioxide and heat stable malts within said amine solution, a second analytical cell for measuring the conductivity of said second liquid stream containing said hydrogen sulfide and carbon dioxide without said heat stable salts, a hydrogen sulfide remover for acting on said second liquid stream and removing said hydrogen sulfide thereby to form a third liquid stream, a third analytical cell for measuring the conductivity of said third liquid stream containing said carbon dioxide without said hydrogen sulfide and said heat stable salts and a computing device operable to receive signals from said first, second and third analytical cells, to analyse said measurements of said conductivity of said first, second and third analytical cells and to produce a value for said acid concentration of said amine solution.